1. Field of the Invention
This invention relates to a disk drive motor employed in a disk drive device adapted to turn an optical disk or the like, and more particularly to a motor rotor which is integral with a disk mounting hub which is built in the disk drive motor, and a method of manufacturing the motor rotor.
2. Related Art
A disk drive device has a disk mounting hub which is rotatable with respect to the device body, and it is so designed that the disk drive motor rotates the hub, so that a disk mounted on the hub is turned.
FIG. 5 shows a disk drive motor which is generally used. As shown in FIG. 5, a disk drive motor 1 comprises a stator assembly 2 secured to the device body, and a rotor assembly 3 which is supported in such a manner that it is rotatable with respect to the device body.
The stator assembly 2 includes: a cylindrical bearing holder 21; a stator core 22 fixedly mounted on the outer cylindrical surface of the bearing holder 21 in such a manner the stator core 22 is coaxial with the bearing holder 21; a drive coil 23 which is wound on a plurality of protruded poles of the stator core 22. A plate-shaped frame 24 are radially outwardly extended from the bearing holder 21, and the frame 24 is secured to the device body with fixing screws.
The rotor assembly 3 includes: a rotary shaft 31 which is rotatably supported through a pair of ball bearings 25 and 26 on the inner cylindrical surface of the bearing holder 21; and a cylindrical disk mounting hub 32 which is secured to the end the rotary shaft 31. The hub 32 includes an annular-yoke-mounting cylindrical portion 33. An annular yoke of magnetic material is fixedly secured to the inner cylindrical surface 33a of the cylindrical portion 33. An annular drive magnet 35 is fixedly secured to the inner cylindrical surface of the annular yoke 34. The drive magnet 35 coaxially surrounds the stator core 22 in such a manner that there is a slight gap between the annular drive magnet 35 and the stator core 22.
The hub 32 has an annular stepped surface 32a, on which a disk is placed (mounted). A plurality of disks (not shown) are stacked through a spacer on the stepped surface 32a, and all of the disks thus stacked are clamped onto the hub 32 with a damper (not shown).
The hub 32, the annular yoke 34, and the drive magnet 35 form a motor rotor 4. The motor rotor 4 is manufactured as follows:
As shown in FIG. 4, first of all, non-magnetic metal material such as aluminum is subjected to die forging, to form a hub manufacturing primary blank 32A. The primary blank 32A is subjected to die forging again, to form a secondary blank which is almost similar to the final product, namely, the aimed hub. The secondary blank is subjected to finishing twice, to obtain the final product, namely, the aimed hub. On the other hand, a magnetic metal plate is subjected to pressing, to form the annular yoke 34.
The annular yoke 34 thus formed is inserted into the cylindrical portion 33 of the hub through the opening, and then the annular yoke 34 is bonded to the inner cylindrical surface of the cylindrical portion 33 with an adhesive agent. Thereafter, the drive magnet 35 is inserted into the annular yoke 34, and then the drive magnet 35 is fixedly bonded to the cylindrical surface of the annular yoke 34 with an adhesive agent. As a result, a motor rotor 4 is obtained in which the annular yoke 33 and the drive magnet 34 are fixedly secured to the hub 32. A motor rotor which is similar to that which has been described above has been disclosed by U.S. Pat. No. 5,517,374.
On the other hand, a method is also known in the art in which, after the hub 3, the final product, has been obtained, the annular yoke 34 is secured to the hub 32 without use of the adhesive agent. That is, in the method, the annular yoke 34 is inserted into the hub 32, and then the annular yoke 34 is fixedly secured to the hub 32 by punching.
As was described above, the yoke 34 is bonded to the hub 32 with an adhesive agent. This method suffers from the following problems:
A first problem is as follows: A clearance (gap) must be provided between the inner cylindrical surface of the cylindrical portion of the hub 32 and the outer cylindrical surface of the yoke 34, and the clearance thus provided must be filled with the adhesive agent. Because of the clearance, when the yoke 34 is bonded to the cylindrical portion of the hub, the central axis of the hub cylindrical portion 33 may be shifted from the central axis of the yoke. If this shift occurs, the motor rotor becomes poor in the balance of rotation; that is, the run-out due to rotation is increased. This makes it difficult to increase the density of data of the disk.
A second problem attributes to the employment of the adhesive agent. That is, when the adhesive agent is not completely solidified yet, it may scatter as gas, which may stick onto the recording surface of the disk.
A third problem is as follows: The annular end face 34a of the yoke 34 is exposed in the opening of the hub cylindrical portion 33. Therefore, cutting oil, cleaning oil, etc. used in the finishing work may enter the space between the inner cylindrical surface of the cylindrical portion and the outer cylindrical surface of the yoke through the end face 34a and stay therein. Hence, during the operation of the motor, a liquid such as cutting oil and cleaning oil oozes out, thus sticking onto the recording surface of the disk.
On the other hand, the method in which the hub is secured to the annular yoke by punching suffers from the following problems. That is, the hub, which is the final product which has been finished by machining, is subjected to punching. Hence, the hub may be distorted by the punching stress. If the hub is distorted, then the hub becomes poor in the balance of rotation, and therefore it is necessary to use a balancer to adjust the balance of rotation.